freecodecamp--freecodecamp/curriculum/challenges/english/blocks/daily-coding-challenges-javascript/6a22d77ddf034bc4e35b1d5d.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
6a22d77ddf034bc4e35b1d5d	Challenge 356: Magic Square Solver	28	challenge-356

--description--

Given a 3x3 grid with one missing number (represented as 0), return the missing number that completes the magic square, or "impossible" if no valid number exists.

A magic square is a grid where every row, column, and diagonal adds up to the same number.

--hints--

solveMagicSquare([[2, 7, 6], [9, 0, 1], [4, 3, 8]]) should return 5.

assert.equal(solveMagicSquare([[2, 7, 6], [9, 0, 1], [4, 3, 8]]), 5);

solveMagicSquare([[0, 14, 12], [18, 10, 2], [8, 6, 16]]) should return 4.

assert.equal(solveMagicSquare([[0, 14, 12], [18, 10, 2], [8, 6, 16]]), 4);

solveMagicSquare([[12, 17, 16], [19, 0, 10], [14, 13, 18]]) should return "impossible".

assert.equal(solveMagicSquare([[12, 17, 16], [19, 0, 10], [14, 13, 18]]), "impossible");

solveMagicSquare([[15, 35, 31], [43, 27, 11], [23, 19, 0]]) should return 39.

assert.equal(solveMagicSquare([[15, 35, 31], [43, 27, 11], [23, 19, 0]]), 39);

solveMagicSquare([[26, 41, 14], [47, 35, 0], [32, 29, 44]]) should return "impossible".

assert.equal(solveMagicSquare([[26, 41, 14], [47, 35, 0], [32, 29, 44]]), "impossible");

--seed--

--seed-contents--

function solveMagicSquare(grid) {

  return grid;
}

--solutions--

function solveMagicSquare(grid) {
  const flat = grid.flat();
  const zeroIndex = flat.indexOf(0);
  const row = Math.floor(zeroIndex / 3);
  const col = zeroIndex % 3;

  const target = grid.find(r => !r.includes(0))
    .reduce((sum, n) => sum + n, 0);

  const missing = target - grid[row].reduce((sum, n) => sum + n, 0);
  const filled = grid.map((r, ri) =>
    r.map((n, ci) => (ri === row && ci === col) ? missing : n)
  );

  const check = (arr) => arr.reduce((sum, n) => sum + n, 0) === target;

  for (let i = 0; i < 3; i++) {
    if (!check(filled[i])) return "impossible";
    if (!check(filled.map(r => r[i]))) return "impossible";
  }
  if (!check([filled[0][0], filled[1][1], filled[2][2]])) return "impossible";
  if (!check([filled[0][2], filled[1][1], filled[2][0]])) return "impossible";

  return missing;
}